As is well known, flat panel displays (FPDs) have become mainstream as image display devices in recent years, the FPDs being typified by a liquid crystal display, a plasma display, an OLED display, and the like. Further, reducing the weight of those FPDs has been promoted. Therefore, currently, thinning of glass substrates used for the FPDs (forming the glass substrates as glass films) is promoted.
Further, there is a growing use of an OLED as a plane light source, such as a light source for interior illumination, which emits only monochrome (for example, white) light, unlike a display that uses TFTs to blink light of three fine primary colors. Further, when an illumination device of this type includes a glass substrate having flexibility, a light-emitting surface is freely deformable. Therefore, from the viewpoint of ensuring sufficient flexibility, there is also promoted further thinning of the glass substrate to be used for the illumination device.
In this context, generally employed techniques for cleaving the glass substrates used for the FPDs, the illumination device, and the like include cleaving the glass substrate with a bending stress generated with respect to a scribe line formed at a predetermined depth in a surface of the glass substrate along a preset cleaving line.
However, it is significantly difficult to form the scribe line with respect to the glass substrate thinned to become a state of a glass film. Therefore, it is difficult to employ such a method of cleaving the glass substrate with a bending stress. Further, defects formed in cleaved surfaces (for example, lateral cracks) may cause a problem of marked deterioration in strength of glass.
In addition, when the glass film is required to be continuously cleaved while being successively conveyed, there is also a problem that it is difficult to continuously cause a bending stress to act on the scribe line formed with respect to the glass film.
As a countermeasure, instead of the cleaving method for a glass film, in which the above-mentioned bending stress is used, cleaving methods for a glass film, in which a thermal stress is used, are currently employed.
Specifically, as described in Patent Literature 1, there is proposed a method in which widthwise end edge portions of a belt-like flat glass are locally heated by a laser and cooled by a cooling device so as to generate a thermal stress, and the end edge portions are continuously cleaved by the thermal stress thus generated.
Further, according to Patent Literature 1, an advancing direction of the end edge portions divided from a body of the flat glass (available glass portion) as a result of cleaving the flat glass is changed perpendicularly downward in a horizontal zone. The end edge portions are cleaved in width directions at lower ends thereof to be discarded, and the available glass portion of the flat glass is conveyed in a horizontal direction as it is without being changed in advancing direction. After that, the available glass portion is cleaved in a width direction by a predetermined length. In this way, glass sheets as products are obtained.
Meanwhile, according to Patent Literature 2, a continuous glass ribbon is cut along predetermined lines by a thermal stress generated by application of a laser, and end edge portions of the glass ribbon thus cut are rotationally supported by a plurality of rollers and conveyed in a direction of being gradually spaced apart from a main portion of the glass ribbon toward a widthwise outer side. In this way, the end edge portions and the main portion are separated from each other.